LT3573EMSEPBF LINER [Linear Technology], LT3573EMSEPBF Datasheet - Page 17

• Manufacturer Part Number: LT3573EMSEPBF
• Description: Isolated Flyback Converter without an Opto-Coupler
• Manufacturer: LINER [Linear Technology]

applications inForMation
Zener clamps. Among them, RCD is widely used. Figure 9
shows the RCD snubber in a flyback converter.
A typical switch node waveform is shown in Figure 10.
During switch turn‑off, the energy stored in the leakage
inductance is transferred to the snubber capacitor, and
eventually dissipated in the snubber resistor.
The snubber resistor affects the spike amplitude V
duration t
t
to the output voltage sensing.
The previous steps finish the flyback power stage design.
5. Select the feedback resistor for proper output
Using the resistor Tables 1‑4, select the feedback resis‑
tor R
SP
voltage.
2
1
is about 150ns. Prolonged t
FB
L I
S PK SW
, and program the output voltage to 5V. Adjust the
Figure 9. RCD Snubber in a Flyback Converter
2
SP
+
–
, the snubber resistor is adjusted such that
f
C
=
V V
C
R
D
(
C
L
S
−
R
3573 F09
N V
•
SP
OUT
may cause distortion
)
C
and
R
voltage. R
A small capacitor in parallel with R
during the voltage spike, however, the capacitor should
limit to 10pF . A large capacitor causes distortion on volt‑
age sensing.
6. Optimize the compensation network to improve the
The transient performance is optimized by adjusting the
compensation network. For best ripple performance, select
a compensation capacitor not less than 1nF , and select a
compensation resistor not greater than 50k.
7. Current limit resistor, soft-start capacitor and UVLO
Use the current limit resistor R
limit if a compact transformer design is required. Soft‑start
capacitor helps during the start‑up of the flyback converter.
Select the UVLO resistor divider for intended input opera‑
tion range. These equations are aforementioned.
TC
transient performance.
resistor divider
resistor for temperature compensation of the output
V
IN
REF
Figure 10. Typical Switch Node Waveform
V
C
is selected as 6.04k.
t
SP
NV
OUT
LIM
REF
to lower the current
filters out the noise
LT3573
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